Scanning Tunneling Microscopy Visualization of Polaron Charge Trapping by Hydroxyls on TiO2(110)

Using scanning tunneling microscopy (STM), we investigate the spatial distribution of the bridging hydroxyl (OHb) bound excess electrons on the rutile TiO2(110) surface and its temperature dependence. By performing simultaneously recorded empty and filled state imaging on single OHbs at different temperatures in STM, we determine that the spatial distribution of the OHb bound excess electrons retains a symmetric four-lobe structure around the OHb at both 78 and 7 K. This indicates that OHbs are much weaker charge traps compared to bridging O vacancies (Ob-vac). In addition, by sequentially removing the capping H of each OHb using voltage pulses, we find that the annihilation of each OHb is accompanied by the disappearance of some lobes in the filled state STM, thus verifying the direct correlation between OHbs and their excess electrons.


■ INTRODUCTION
A Polaron is a quasiparticle formed when an electronic charge carrier introduced into a dielectric becomes localized at one of the symmetrically equivalent sites available.This alters the equilibrium positions of the surrounding lattice ions and subsequently creates a potential well that traps the carrier. 1 These self-trapped polarons are believed to play a vital role in the physics and chemistry of many metal oxides, and technologically relevant phenomena as diverse as photolysis, 2 high temperature conductivity 3 and resistive switching. 4−35 Titanium dioxide (TiO 2 ), a prototypical metal oxide system with applications ranging across heterogeneous catalysis, photolysis and solar cells etc., 36−40 has recently become a realistic material platform with which to study the polaron properties and their relevance to chemical processes.Taking the most stable (110) face of TiO 2 in the rutile form as an example: its surface structure (Figure 1a) comprises rows of fivefold coordinated Ti 4+ ions that alternate with those of twofold coordinated bridging O 2− ions (O b ). 41TiO 2 is a wide band-gap insulator (E gap ∼ 3 eV), which can be made semiconducting upon reduction by cycles of ion sputtering and annealing. 42Such a reduction process leads to the formation of bridging oxygen vacancies (O b -vacs) on the surface, 43−45 and two excess electrons for each created O b -vac.Previous studies showed that these excess electrons mainly reside at the subsurface Ti 6c sites (beneath the surface Ti 5c rows) surrounding the O b -vacs and reduce the associating Ti ions, 30,46−49 with a small number occupying the surface Ti sites as observed by resonant photoemission diffraction. 50,51This results in Ti 3+ 3d derived defect states, namely the band gap state (BGS), formed at ∼1 eV below the Fermi level (E F ) within the band gap. 52,53Further studies verified the polaronic character of the O b -vac bound excess electrons, 54−56 and their strong interaction with adsorbates in model chemical processes. 31,57,58 b -vacs on TiO 2 (110) are the most reactive sites for a diverse set of chemical reactions.Scanning tunneling microscopy (STM), resonant photoemission diffraction, and density functional theory (DFT) calculations have been widely used to study the excess electron distribution in different metal oxide systems owing to their complementary advantages.In particular, using simultaneously recorded empty-(ES) and filled-states (FS) imaging (or dualmode imaging) in STM, we previously observed that the O bvac bound electron polaron distribution adopts a symmetric four-lobe structure surrounding the O b -vac at 78 K, which transforms into one of the three in-equivalent two-lobe structures as the temperature drops to 7 K. 56 Here, we use low temperature dual-mode imaging in STM to determine how the bound polarons are distributed around the OH b species following dissociative H 2 O adsorption.Moreover, we investigate their temperature dependent behavior.The answers to these questions will further our understanding of the intrinsic difference between O b -vac and OH b species as charge traps.It will also illuminate the debate about the difference between the two types of OH b (one formed at the O b -vac site and another at one of the neighboring O b site).60 ■ EXPERIMENTAL SECTION STM experiments were performed using an Omicron GmbH low temperature scanning tunneling microscope housed in an ultrahigh vacuum chamber with a base pressure in the 10−11 mbar region.To probe excess electrons associated with OH b , we performed simultaneously recorded filled (FS, using negative samples bias) and empty states (ES, using positive sample bias) STM imaging (namely, dual-mode imaging): in the forward scan along the fast scan direction, a line of topography data is recorded at positive sample bias; in the backward scan, a line of data is recorded with negative sample bias so that two images (ES and FS images) are recorded quasi-simultaneously.This eliminates the effects of thermal or piezo drift so that images obtained at opposite polarities can be directly correlated.To rule out the possibility of introducing any artifact from the forward scan to the backward scan, the polarity was occasionally reversed, i.e. the forward scans were negatively biased and backward scans were positively biased.No difference was observed in the resulting images.
To obtain a TiO 2 (110) single crystal sample with sufficient electrical conductivity for STM measurements at very low temperatures (T ∼ 7 K), we employed a special sample preparation procedure: first, a fresh rutile TiO 2 (110) sample (Pi-Kem) was subjected to about a hundred cycles of argon ion sputtering and vacuum annealing up to 1000 K; then, the asprepared sample was left in the preparation chamber at a base pressure of 2 × 10 −10 mbar at room temperature.−61 In this way, a fully hydroxylated surface (h-TiO 2 ) with a high density of OH b s is formed.This reaction removes all the surface O b -vacs. 64e previously showed that the O b -vac bound polarons separated from each other by at least three unit cells along the [001] direction, or at least one unit cell along [1̅ 10] have no The Journal of Physical Chemistry C measurable interaction with each other. 56On this basis, we prepared single OH b s on h-TiO 2 as follows: first using dualmode imaging to locate the OH b s isolated from regions of charged impurities.Then, using voltage pulses (3 V, 1 ms at 78 K; 3.5 V, 1 ms at 7 K) we removed the capping Hs of all other OH b s surrounding our targeted OH b species. 65This led to a small surface area, usually about (5 nm)-2 containing only a few single noninteracting OH b s with their associated excess electron distributions.

■ RESULTS AND DISCUSSION
Figure 1 shows a dual-mode 78 K image of a single OH b on TiO 2 (110) (Figure 1b,d), and those taken after the removal of its capping H (Figure 1c,e) using a +3 V, 1 ms tip pulse.Before the capping H removal, the FS image of a single OH b is characterized by a bowtie-shaped feature at its position linking the neighboring Ti 5c rows, altogether with a nearly symmetric four-lobe structure with lobes located at the diagonal Ti 5c sites (Figure 1d).All of these features disappear after the capping H is removed (Figure 1e).Previous STM work by Minato et al. observed a similar FS image of single OH b . 52Previous DFT calculations of the hydroxylated TiO 2 (110) surface show that the Ti 6c sites in the second subsurface layer beneath the surface Ti 5c rows are the most stable sites for the OH b -polaron occupation. 46,66On this basis, we attribute the observed enhanced contrast along the Ti 5c rows in the FS images to the excess electrons populating in the second subsurface layer underneath the surface Ti 5c rows.
We previously reported that the spatial distribution of the O b -vac bound excess electrons on the reduced surface of TiO 2 (110) (r-TiO 2 ) transforms from a symmetric four lobe structure at 78 K into one of three asymmetric, two lobe structures at 7 K. 56 Our findings confirmed the polaronic nature of the excess electrons on TiO 2 (110), which motivates this study of the spatial distribution of the OH b bound excess electrons and its temperature dependence.Before looking into this, we first examined how the FS image contrast changes when the capping Hs of a group of OH b s are removed by using tip pulses.The results are shown in Figure 2, where all of the capping Hs at the center of the imaged region (marked by dashed rectangles) are removed (Figure 2a).In the FS images (Figure 2b), the H-stripped area appears much darker along the Ti 5c rows compared with the H-capped region.Hence, there is a direct correlation between OH b s and the excess electrons that appear as lobes on the Ti 5c rows in the FS images (Figure 2b).There are two likely modes in which the excess electrons could dissipate, depending on whether H is desorbed as a cation or a neutral species.In the former, electrons would be lost to the STM apparatus, while in the latter, the electrons would be captured by the bridging O ions.
Having established the relationship between OH b s and their associating excess electrons, we turn to the 7 K distribution of excess electrons surrounding single OH b species.Figure 3 shows the dual-mode images recorded before and after the capping Hs of two single OH b s was sequentially removed using +3.5 V, 1 ms tip pulses.The ES images in Figure 3a−c simply evidence the conversion of OH b to O b .In the FS image (Figure 3d), each OH b image appears to be characterized by a nearly symmetric four-lobe structure.This is very different from the behavior of O b -vacs, the excess electron distribution of which is highly asymmetric at 7 K. 56  Not only is there little difference between the spatial distribution of the OH b bound excess electrons at 78 and 7 K, but there is also a similar effect of removing capping H at the two temperatures.Figure 3 shows the dual-mode images of two separated single OH b s, and those recorded after the sequential removal of their capping Hs by tip pulses.After the capping H on the left is removed (Figure 3b), not only the bowtie-shaped feature in the FS STM at the OH b center disappears, the lobes distributed at the Ti 5c sites around the OH b (Figure 3d) also vanish in the FS image (Figure 3e).The similar observation also applies to the OH b on the right (see Figure 3e,f).To better visualize the changes in the FS images, we present in Figure 3g,h, the difference images formed by subtraction of the FS images taken before and after each capping H removal. There, one clearly can see that each OH b is characterized by a bowtie-shaped feature at the center with four lobes distributed at each of the second nearest Ti 5c sites around it.This again confirms the observation of a nearly symmetric, four-lobe structure for the distribution of the OH b -bound excess electrons at 7 K. Taking a closer look at the difference image (Figure 3g), we also observe a redistribution of the excess electrons in the vicinity of the OH b on the right after the capping H of the OH b on the left is removed, as evidenced by the additional lobe of density loss in the bottom region between the two OH b s.In addition, the difference images (Figure 3g,h) show only a reduction in the FS contrast in close proximity to the OH b s, while that in the surrounding region remains unchanged.This is consistent with dissipation of the excess electrons through the STM apparatus or capture by bridging O ions, as noted above.
Previous studies showed that when a H 2 O molecule adsorbs dissociatively at an O b -vac, two OH b s, one at the O b -vac site (namely v-OH b ) and another at one of the two nearestneighboring O b ions (namely b-OH b ), are formed. 59,62A later STM study by Zhang et al. determined that the capping Hs of b-OH b s are ten times more likely to hop along the Ti 5c rows compared to v-OH b s, evidencing their inequivalence. 60One possible scenario is that the distribution of the excess electrons,   Figure 4 shows a series of simultaneously recorded dualmode images, recorded at 6.6 K, taken before and after the sequential removal of each of the capping Hs with tip pulses (+3.5 V, 1 ms).Before imaging, the capping Hs of most of the OH b s originally present in the scanned region were removed using the same tip pulses.This leaves only five OH b s and one OH b pair remaining in the scanned region.As shown in the FS images (Figure 4f−j) and in the difference images (Figure 4k− n), the removal of each capping H is always accompanied by changes in image contrast in the FS image.Taking the OH b at the bottom right as an example, after its capping H is removed (Figure 4b), the two lobes originally present at the Ti 5c sites above that of OH b disappear (Figure 4f).Their disappearance is also accompanied by some increase in the intensity of the lobes at the Ti sites above the OH b pair at the top right of the image (Figure 4g).After the first capping H within the OH b pair was removed by a tip pulse (Figure 5b), the lobes become significantly weaker in intensity and displace away from their original positions (Figure 5e,g).Then, after the second capping H was removed (Figure 5c), the lobes further weaken and dissipate further away from the original position of the OH b pair (Figure 5f,h).Based on the above, we conclude that, first, OH b s are weaker as charge traps compared to a OH b pair, and second, as all the charge traps on the surface are removed, the excess electrons originally bound to those charge traps are dissipated.Meanwhile, the resulting absence of any charge traps leads to much more uniform appearance along the Ti 5c rows (Figure 5f).
Through comparison of the STM data shown in Figures 3−5, we find that the almost symmetric four-lobe structure of the spatial distribution of the OH b bound polarons at T = 7 K transforms into one of the asymmetric two-or three-lobe structures as temperature is reduced to 6.6 K.We attribute such change in the polaronic distribution to the temperaturedependent hopping behavior of polarons: at 7 K polarons still hop between the subsurface Ti 6c sites surrounding a OH b and their motion starts to freeze; at 6.6 K their motion becomes completely frozen and depending on the local chemical environment, 56 their distribution about a OH b adopts one of the asymmetric structures.

■ SUMMARY
To summarize, employing dual-mode imaging to study the spatial distribution of the OH b bound excess electrons on the (110) surface of TiO 2 rutile, we found that their distributions retain a symmetric, four-lobe structure at temperature of 7 K, suggesting that OH b s are much weaker as charge traps compared to O b -vacs, with their associated polarons requiring much less energy to hop over different Ti sites surrounding the The Journal of Physical Chemistry C vacancies.In addition, using voltage pulses to sequentially remove the capping H of each of the OH b s and monitoring the corresponding changes in the image contrast within the FS STM, we found that every capping H removal is accompanied by the disappearance of some FS contrast surrounding the removed capping H position, thus verifying that each OH b , once formed, is accompanied by a polaron.

Corresponding Author
Geoff Thornton − Department of Chemistry and London Centre for Nanotechnology, University College London, London WC1H 0AJ, U.K.; orcid.org/0000-0002-1616-5606;Email: g.thornton@ucl.ac.uk Taking H 2 O adsorption on TiO 2 (110) as an example: H 2 O molecules adsorb dissociatively at O b -vacs, forming a pair of bridging hydroxyls (OH b ) for each O b -vac. 59−63 Over time, the OH b s within the OH b pair diffuse away from each other and form two single OH b s.Previous studies showed that dissociative H 2 O adsorption on TiO 2 (110) does not cause any change to the BGS population.On this basis, one can assume that upon dissociative H 2 O adsorption, the excess electrons originally belonging to O b -vacs are transferred to the newly formed OH b pairs, with each pair sharing two excess electrons.Also, it is believed that further splitting of a OH b pair into two single OH b s should lead to a redistribution of the excess electrons between the two OH b s.

Figure 1 .
Figure 1.(a) Structural model of rutile TiO 2 (110).Spheres of different colors represent O b s (cyan), in-plane Os (mid blue), subsurface Os (dark blue), surface Ti ions (mid red), sub-surface Ti ions (dark red), and H atoms (pale pink), respectively.O b -vac and OH b species are also indicated.(b) Empty-and (d) filled-states 78 K STM images of (4 nm) 2 TiO 2 (110) with a single OH b .In (d), a bowtie feature at the OH b site that links the adjacent Ti 5c rows is indicated (c,e) As (b,d), recorded after removal of the capping H atom (marked by an arrow) from the OH b with a 3 V, 1 ms tip pulse.Scan parameters (V, I): (b,d) 0.9 V, 50 pA, and (c,e) −1 V, 50 pA.Dashed rectangles mark the region where the four-lobe excess electron distribution surrounding the OH b (d) disappears after the capping H removal.
We attribute this difference to the absence of polaron hopping at low temperature in the case of the O b -vac bound electrons but not for the OH b polarons.Intuitively, the much faster hopping of the OH b bound electrons evidenced at 7 K can be understood by (i) the weaker attractive force of OH b (formal charge of 1+) to electrons as compared to O b -vac (formal charge of 2+), and (ii) the much smaller local distortion of the lattice from the formation of an OH b by adding a H to an O b as compared to that of O b -vac (by losing an O b ).

Figure 2 .
Figure 2. (a) ES-and (b) FS-images of h-TiO 2 recorded after sequential removal of the capping Hs of all OH b s in the central part of the scanned region using +3.5 V, 1 ms tip pulses.The images were recorded at 7 K. Image size: (15 nm) 2 .Scan parameters (V, I): (a) +2 V, 10 pA; (b) −2 V, 1 pA.

Figure 3 .
Figure 3. Simultaneously recorded (a) ES and (d) FS images of TiO 2 (110) containing two single OH b s.The images were recorded at 7 K. (b,e) As (a,d) recorded after the capping H of the OH b on the left was removed by a +3.5 V, 1 ms tip pulse.(c,f) As (b,e) recorded after removal of the capping H of the OH b on the right.Circles mark the OH b positions.Image size: (2.74 nm) 2 .Scan parameters (V, I): (a−c) +2 V, 30 pA; (d−f) −2 V, 1 pA.(g−h) Difference images formed by subtraction of the FS image in (d) from that in (e), and of the FS image in (e) from that in (f), respectively.

Figure 4 .
Figure 4. Simultaneously recorded (a) ES and (f) FS images of h-TiO 2 .Before imaging, the capping Hs of most OH b s originally present were removed using +3.5 V, 1 ms tip pulses, leaving only five OH b s and one OH b pair remaining in the imaged region.(b−j) As (a−f) following the sequential removal of the capping H of each of the OH b species using the same tip pulses.Solid circles mark the positions of single OH b s.Open circles mark those in the OH b pair.Arrows indicate the capping H being removed in each frame.All images were recorded at 6.6 K. Image size: (4 × 4) nm 2 .Scan parameters (V, I): (a−e) +2 V, 30 pA; (f−j) −2 V, 1 pA.(k−n) Difference images formed by subtraction of the FS images obtained before and after the removal of the capping H within each OH b species.
Similar changes to the excess electron distributions surrounding the OH b and OH b pair on the surface have also been observed following the removal of other capping Hs, see the images (Figure 4g−j) and the corresponding difference images (Figure 4l−n) for such changes.In addition to studying the influence of neighboring OH b s on the polaron distribution surrounding a OH b pair, we have also investigated how the polaron distribution surrounding an OH b pair changes upon the sequential removal of its capping Hs, and our results are shown in Figure 5.The initial empty and filled state images are shown in Figure 5a,d, respectively.The filled state image evidences a distribution of the OH b pair bound polarons that has a three lobe structure.The apparent asymmetry in the FS image is consistent with the asymmetric behavior observed by Zhang et al. in the mobility of the two types of bridging hydroxyls.

Figure 5 .
Figure 5. Simultaneously recorded (a) ES and (d) FS images of TiO 2 (110) containing one single OH b and one OH b pair.(b−c) As (a), but recorded after the capping H atoms of the OH b pair were removed sequentially using +2.6 V, 200 ms tip pulses.Solid circles mark the positions of single OH b .Open circles mark the OH b within the OH b pair.Arrows indicate the capping H that was removed in each frame.(e−f) Corresponding FS images of (b−c), respectively.All images were recorded at 6.6 K. Image size: (4 × 4) nm 2 .Scan parameters (V, I): (a−c) ±2 V, 30 pA; (d−f) −2 V, 1 pA.(g−h) Difference images formed by subtraction of the FS images obtained before and after the removal of each capping H within the OH b pair.